Rotary joint utilizing a fluid slip ring



Dec. 9. 1969 R. M. GOODMAN, JR

ROTARY JOINT UTILIZING A FLUID SLIP RING Filed Nov. 7, 1966 5 w M 0 ohNb 3 mh w% w m Va 0 I W. wfl .l m I u lll i e .I l! f l I ali m \w *h Nw P B NW N N NW QN N N\ X sm ww sivw 5 Wm x United States Patent US. Cl.174-21 3 Claims ABSTRACT OF THE DISCLOSURE An improved coaxial rotaryjoint for passing a wide range of frequencies including separate pathsfor high and low frequencies, the low frequency path including a mercuryslip ring between the inner conductors connected at the joint. The slipring comprises a recess in one of the inner conductors and projection onthe other extending into the recess, with an O-ring seal around the neckof .the projection. Mercury is in the portion of the recess not ccupiedby the projection. To keep mercury in the portion of the recess in anarrow space between the projection and the wall of the recess, anexpandable element is placed in the recess, which expands to occupy anyspace not filled with mercury.

This invention relates to electrical rotary joints and, in particular,to improved means for sealing a fluid (such as mercury) slip ring whichmay be used in electrical rotary joints. This invention also relates toimproved means for passing RF or high frequency 1000 mc. to 12 go. forexample) energy through the inner conductors of a rotary joint.

This invention is an improvement of the copending application by thesame inventor Ser. No. 592,402, filed Nov. 7, 1966, now Patent No.3,426,309, entitled Rotary Joint and assigned to the same assignee asthe present invention.

As has been discussed in the above-mentioned copending application, aserious problem arises in rotary joints which employ an electricallyconducting fluid, such as mercury, as a slip ring because unacceptablyhigh friction torque tends to develop between the seal for the fluid andthe rotating members. In certain applications, such as where a rotaryjoint is employed to connect a rotating coaxial line to a stationaryone, the high friction torque must be resisted by the outer conductor ofthe stationary coaxial line. However, these outer conductors are notdesigned to withstand high rotational stress and, hence the torquedeveloped at the seal cannot become too high.

It is a primary purpose of this invention to provide an improved sealfor fluid (such as mercury) slip rings employed in coaxial ornon-coaxial rotary joints.

It is further the object of this invention to provide an improvedcoaxial rotary joint having a fluid (such as mercury) slip ring wherethe seal for the mercury develops a minimum amount of frictional torquebetween the stationary and rotating members.

It is a further object of this invention to provide an improved RF orhigh frequency path through the inner conductor of a coaxial rotaryjoint.

Other objects and advantages of this invention will become apparent uponreading the appended claims in conjunction with the following detaileddescription and the attached drawings, which:

FIGURE 1 is a cross section view of a rotary joint in accordance withthe principles of the invention; and

FIGURE 2 is a detail portion of FIGURE 1.

Inasmuch as most of the mechanical details of the general type of rotaryjoint disclosed in FIGURE 1 are well known and since they have beenthoroughly described in the above-mentioned copending application ofappli- 3,483,307 Patented Dec. 9, 1969 cant, the description of themechanical features will only be briefly described in this application.The rotary joint is generally indicated at 10. It incorporates a housing12, a stationary inner conductor or a first cylindrical inner conductor14, a rotating inner conductor or second cylindrical inner conductor 16,and a rotating outer conductor or second cylindrical outer conductor 18.Housing 12 and connector assembly member 22 comprise a stationary outerconductor or first cylindrical outer conductor. The first cylindricalouter conductor may, for ease of manufacture, be produced as anassembly. Although this invention is described in terms of certainmembers rotating and oiher members being stationary, it will beunderstood by those having ordinary skill in this art that theprinciples of this invention are applicable whenever relative rotationoccurs between the outer and inner conductors 12 and 14 respectively,and the outer and inner conductors 18 and 16 respectively. Connectorsleeve 20 receives a male member (not shown), sleeve 20 being secured toconnector assembly member 22 via retaining ring 24. The outer conductorof the stationary coaxial line (not shown) connects to connecting member26 and thus an electrical path is provided via member 26 and connectorassembly member 22 to (l) mercury bath 28, thereby providing a DC andlow frequency path in the outer conductor, and (2) conducting member 30via gap 32 thereby providing a capacitive, RF path from the stationaryto the rotating conductor. The RF path in the outer conductor iscompleted to rotating outer conductor 18 since member 30 is inelectrical contact therewith, member 30 being fixedly attached toconductor 18. The gap 32 is preferably 0.001.

The DC path in the outer conductor through mercury slip ring 28 has beenmentioned above. Recess 34 formed within member 18 provides a receptablefor the mercury, further, the receptacle may be provided in member 22 orin members 22 and 18. The mercury is sealed between stationary outerconductor 22 and rotating outer conductor portion 18 by sealing membersor O-ring seals 36 and 38. These rings are commercially available and aring that has been successfully employed in this invention is O-ring No.2-16 manufactured by Parker Seal C0. Other types of squeeze type moldedseals can also be used. However, O-rings are preferred.

In order to prevent leakage through the seal of a mercury slip ring of arotary joint, it is required that the seals be statically anddynamically fluid tight. A fluid tight seal is one which has no passagethrough which the fluid might escape. Squeeze type molded seals such asO-rings properly designed and installed have this fluid tightcharacteristic. In rotating seals, however, and especially in high speedrotating seal applications, certain problems arise which have causedrotating seals utilizing squeeze type molded seals (O-rings) to beconsidered impractical. These problems include the Joule effect whichcauses O-rings to contract when heated while under tensile stress suchas that due to the installation. In an effort to counteract thediificulties arising from this problem, it is known to those skilled inthis art that certain dimensions must be specified for rotary O-ringseals which will produce peripheral compressive O-ring stress due to theinstallation. Seals manufactured to these dimensions, however, are stillunsatisfactory as high speed seals for mercury slip rings, this beingevidenced by unacceptably high friction torque (at all speeds), heat andwear which eventually result in failure of the seal. This problem isparticularly severe when the rotational friction must be resisted bytorsional stress in a coaxial cable.

In the seals 36 and 38, which have been successfully employed to seal amercury slip ring, low friction torque is present at all speeds, nooverheating occurs at high speeds, and a fluid type seal with long liferesults. The improvements result from first inserting the O-rings 36 and38 respectively into O-ring gland grooves 44 and 46 which provideperipheral compression as has been mentioned above to minimize the Jouleeffects. After insertion into grooves 44 and 46, the inner diameter ofthe O-rings (assuming that Parker No. 2-16 O-rings are used) isdecreased to a preferred predetermined value (for example, an innerdiameter of 0.603" to 0.604"). The rotating shaft inner diameter (thediameter across member 30) is established at a second accuratelypredetermined value (for example, 0.606" to 0.607") so that a radicalsqueeze of a third predetermined value (for example, 0.001" to 0.003")results when the rotating shaft is inserted through the installedO-ring. Broadly speaking, the amount of radial squeeze required isdirectly related to the cross section diameter of the O-ring.

In the case of the Parker No. 2-16 the cross section diameter of theO-ring is 0.070; thus, in the above example the radial squeeze of theshaft, compressing the installed O-ring in a radially outward direction,equals 1.4% to 4.3% of the O-ring cross section diameter. Theconventional shaft diameter for the rotary O-ring seal recommended bythose skilled in the art would have resulted in a radial squeeze of 7.9%to 9.3%. Tests using the recommended dimensions were unsuccessful inthis application resulting in excess friction torque, overheating, andwear and ultimate failure as a fluid type seal. The O-ring seal squeezedin accordance with the teachings of this invention will functionsatisfactorily with liquid such as mercury in spite of mercurysnon-lubricating properties. This seal would also work for fluids whichdo lubricate if used at moderate fluid pressures and would provide theadvantage of reduced seal torque.

The preferred seal material for the O-ring is nitrile rubber impregnatedwith M03 such as Parker Seal Companys compound No. Nl63-7, which has ahardness of 70 durometer. This compound and hardness has provensatisfactory but obvious variations will occur to those having ordinaryskill in this art. For example, graphite is a useful (although, in thelight of tests, less efficient) solid lubricant with which the O-ringcompound can be impregnated. Other solid lubricants may be used if thematerial is compatible with the other materials (mercury, nickel platedsteel, plastics and brass) with which the lubricant impregnated O-ringwould be in contact. It is also possible that suitable O-ring compoundscould be impregnated with useful non-solid lubricants.

Referring to FIGURES 1 and 2, the inner conductor will now be discussed.The entire area 52 is filled with mercury. In particular, the area 54along length A is filled with mercury. This is done in order to minimizethe effect of the inner conductor joint to RF signals, as will be moreapparent hereinafter. The low frequency and DC signals flow from thestationary to the rotating members of the inner conductor joint throughthe entire mercury pool, while the RF signals are capacitively coupledfrom hollow cylindrical portion 56 of stationary inner conductor 16 toportion 58 of rotating inner conductor 14 across gap 60. Gap 60 isequivalent to a groove around the inner conductor as long as there areno air pockets along lenth A. This groove must be kept to a minimum. Ifnot, a discontinuity capacitance of sufficient value will occur andimpede the efiicient transfer of RF signal energy. That is, if airpockets exist along length A, then the inner conductor joint may stilloperate as desired because thefair pockets act as small seriescapacitors to the RF signal; however, if the orientation of the rotaryjoint is changed the air pockets will move and their new location andsize may be such that the RF impedance has increased to a point wherethere is poor transfer of RF signal energy through the rotary joint.

In order to insure that the void along length A is com pletely filledwith mercury and that no air bubbles are present, it is necessary thatthe mercury filled cavity be maintained under a constant fluid pressure.In the preferred configuration shown, a unique use of closed cellpolyurethane foam 50 is employed. During installation all air is removedfrom the cavity and as the stationary inner conductor and rotating innerconductor are moved together in an axial direction to achieve the properpositioning of the parts, a cylindrical shaped piece of the closed cellplastic foam is placed under compression compressing the gas within thecells of the foam. This permits the compressed foam to act a a pneumaticspring maintaining fluid pressure within the mercury filled cavity andforcing the mercury into the cylindrical void described above alonglength A.

Thus, the joint of the inner conductor constitutes an improvement overthe joint shown in FIGURE 3 of the beforenientioned copendingapplication. As can be seen from this FIGURE 3 of the copendingapplication and FIGURE 2 of this application, the construction of thejoints is substantially different, the most important difference beingthat the mercury in the instant invention is forced into the annularspace surrounding projection 54 up to a point approximately adjacent gap60'.

The detailed construction of the joint of the inner conductor of FIGURE2 includes a projection 59 which is integrally connected to stationaryinner conductor 14. Member 61 is mounted on projection 59, the members59 and 61 both being electrically conductive. A raised portion 63 onprojection 58 and member 61 act as a retaining member for O-ring 62which seals the mercury bath.

O-ring 62 should also be compressed in a manner similar to that alreadydescribed with respect to seals 36 and 38. Thus, the seal 62 should besqueezed in a direction radial outward from the center axis ofprojection 58 by an amount equal to 1.4% to 4.3% of the cross sectiondiameter of the seal 62, this seal being shown in its squeezed conditionin FIGURE 2.

Referring now to FIGURE 1, the remaining mechanical features of therotary joint will now be described.

Ball bearing assemblies 66 and 68 are disposed between stationaryhousing 12 for rotating inner conductor 18 to facilitate axial andradial alignment therebetween. Annular members 70 and 72 respectivelysupport stationary inner conductor 14 and rotating inner conductor 16.Members 70 and 72 are preferably made from Teflon. Female connector 74is adapted to receive a rotating coaxial line as shown in FIGURE 1.

Clip rings 76 and 78 respectively prevent ball bearing assemblies 66 and68 from axially sliding along rotating outer conductor 18. Spacer washer80 mechanically separates member 22 from ball bearing assembly 66.Static O-ring 81 prevents leakage of fluid into the joint.

Nut 82 is employed to hold the entire assembly together, preventingaxial movement of the rotating and stationary components with respect toeach other. The torque to which nut 82 is tightened affects the spacingof gaps 32 and 60. As the nut 82 is screwed into housing 12, a force isapplied to assembly 68 forcing it against clip ring 7 8. As the nut isfurther screwed into housing 12, the load is transmitted through clipring 76 and ball bearing assembly 66, thereby securing assembly 66against spacer washer 80. As the load is applied to clip ring 76, aslight movement to the left of rotating outer conductor 18 and rotatinginner conductor 16 will take place due to deflections in the affectedcomponents. The width of gap 32 and of gap 60 can be adjustedappropriately by selectively varying the thickness of spacer washer 80.

Set screw 82 is removed whenever recess 34 must be filled with mercury,the position of set screw .82 with respect to recess 34 not beingcritical.

The term DC and low frequency typically includes frequencies from DC to1000 mc. while the term high frequency or radio frequency typicallyincludes frequencies from about 1000 mc. to 12 gc. of course, one havingordinary skill in this art can vary the ranges in accordance with therequirements of a given application.

Numerous modifications of the invention will become apparent to one ofordinary skill in the art upon reading the foregoing disclosure. Duringsuch a reading, it will be evident that this invention has providedunique rotary joint for accomplishing the objects and advantages hereinstated. Still other objects and advantages, and even furthermodifications will be apparent from this disclosure. It is to beunderstood, however, that the foregoing disclosure is to be consideredexemplary and not limitative, the scope of the invention being definedby the following claims.

What is claimed is:

1. A coaxial rotary joint comprising:

a first cylindrical outer conductor;

a second cylindrical outer conductor concentric with said first outerconductor, said first and second conductors being relatively rotatablewith respect to one another;

means for coupling DC and low frequency signals from one of said outerconductors to the other;

means for coupling signals of higher frequencies than said DC and lowfrequency signals from said one outer conductor to said other outerconductor;

a first cylindrical inner conductor having a projection extendingtherefrom;

a second cylindrical inner conductor having a recess;

an electrically conductive fluid bath in said recess for coupling DC andlow frequency signals between said inner conductors;

means mounting said inner conductors for rotation with respect to eachother with said projection in but not filling said recess, and with anannular gap between said first inner conductor and the rim of saidsecond inner conductor adjacent said recess, for coupling said signalsof higher frequencies between said inner conductors;

means sealing between said first and second inner conductors to retainfluid in said recess, there being an annular space between said secondinner conductor and the portion of said projection in said recess beyondsaid sealing means and a reservoir space in said recess beyond thedistal end of said projection, said reservoir space communicating withsaid annular space and compressed means in said reservoir spaceoccupying space in said recess not occupied by said fluid bath andcapable of occupying any space in said reservoir vacated by leakage ofsaid third bath thereby assuring that said annular space will be filledwith said fluid bath.

2. A coaxial rotary joint as set forth in claim 1 in which said meansfor maintaining said electrically conductive fluid in said annular spacecomprises a resilient member in said recess which is compressed whensaid projection is inserted into said recess.

3. A coaxial rotary joint as set forth in claim 2 in which said meansfor maintaining said electrically conductive fluid in said means formaintaining said electrically conductve fluid in said annular spacecomprises closed cell foam plastic.

References Cited UNITED STATES PATENTS 2,424,545 7/1947 Bard.

2,730,602 1/1956 Porterfield 33956 XR 2,890,304 6/1959 Cole.

3,125,649 3/1964 St. Cyr.

DARRELL L. CLAY, Primary Examiner US. Cl. X.R. 17486; 339-5

